Everyone knows that fish live underwater, but a lot of people don’t realize that fish breathe oxygen underwater, just like we do on land. Dissolved oxygen (DO) is critical for fish and other aquatic animals to survive.

Much in the same way that we wouldn’t survive long if our oxygen supply was suddenly cut off, fish and other aquatic animals that are unable to escape areas of extremely low dissolved oxygen will suffocate and die.

Unquity Brook Streambed Dry at Brook Road Culvert (photo by Tom Palmer, Sept. 2016)

Dissolved oxygen data collected by our Citizen’s Water Monitoring Network (CWMN) has shown an alarming trend of plummeting dissolved oxygen levels across the watershed this summer.

While DO does typically decline during the summer, the declines we are seeing this summer are more severe than what has been seen in the recent past, and this is likely due to our abnormally hot July, and the extreme drought we’ve been suffering over the past several months.

CWMN Dissolved Oxygen Data

While watching the animation below its important to note a few things.

• The number of red “trouble” points increases from 1 point in May to 15 points in August.
• The DO levels shift from a minimum value of 3.0 in May to 0.19 in August, and a maximum DO value of 10.2 in May to a value of 7.85 in August.
• Two points (one in Canton at Pequit Brook and the other in Milton at Unquity Brook) disappear from the map in July and August. There is no data for these points because the stream had completely dried up at those locations by mid July.

• Red = DO levels that are stressful to deadly for all aquatic organisms

• Yellow = DO levels that are stressful for cold water species and near stressful for warm water species

• Blue = DO Levels that are healthy for all species

What Affects DO Levels?

In streams and ponds, dissolved oxygen is generated naturally by rapids, waterfalls, or by wind pumping oxygen into the water, and by aquatic plants and algae photosynthesizing underwater. Other factors that affect DO are temperature, salinity and nutrient levels. Cold water can hold more oxygen than warm water, and fresh water holds more oxygen than salt water. High levels of nutrients like phosphorus and nitrogen can lead to a process called eutrophication, which causes the accelerated breakdown of organic matter in the water by microbes that consume oxygen in the process. Eutrophication can be made worse when warm temperatures are combined with excess nutrients.

This Summer’s Drought and Heat Waves are Making Things Worse.

Temperature and precipitation data from the Blue Hills Weather Observatory comparing this summer and last summer. While last year’s summer was abnormally dry, this year was even more extreme. Many climate scientists are predicting that this trend may be the new norm.

The Neponset Watershed, and most of Massachusetts, has been suffering a severe drought for much of the summer. That combined with the abnormally hot July has created terrible conditions for Dissolved Oxygen.

Stream and pond water temperature (and thus DO levels indirectly) is largely controlled by water volume, air temperature, exposure to direct sunlight, and the amount of cold groundwater seepage a water body receives.

Under drought conditions, surface-water levels are lower than normal, leading to hotter than normal water temperatures as shallow waters heat up faster than deep water.

In addition to low surface-water levels, lower groundwater levels, due to drought and groundwater withdrawal, mean streams and rivers are getting less cold spring water contributions that would otherwise help to cool down the surface waters.

Regular rain events flush out ponds of excess nutrients and stagnant water that have been baking in the sun. During a drought this flushing doesn’t occur as often which creates high nutrient, high temperature ponds where eutrophication consumes oxygen and causes DO crashes.

What can be done?

• Conserve Water During the Summer: We can’t control when or how much it rains, but we can control how much water we use. Reducing water use during the summer months by allowing your lawn to go dormant, taking shorter showers, and not washing your car will help to ensure a steady supply of icy cold ground water to local streams which in turn will help keep the fish breathing easy.

• Reduce Fertilizer Use and Properly Dispose of Yard Waste: Lawn fertilizers contain high levels of Phosphorus and Nitrogen, and any excess that isn’t consumed by plants runs off into local water ways and fuels harmful eutrophication.  Grass clippings and fallen leaves are also chock-full of nutrients so properly disposing of them is key to keeping excess nutrients out of our streams, rivers, and ponds.

• Plant Trees Along Exposed Stream Banks: Trees help to shade the water keeping it nice and cool and DO rich while also absorbing excess nutrients and helping to prevent erosion.

• Remove out-of-use Dams: Man-made ponds are fun for recreation, but aren’t great for DO. Ponded water bakes in the sun raising the water temperature downstream. Ponds also collect and concentrate excess nutrients which can fuel eutrophication and cause hazardous cyanobacteria blooms.

–Chris Hirsch, Environmental Scientist, Sept. 2016

For more information, contact Chris at hirsch@neponset.org or 781-575-0354 x302